In general, a superconducting coil is cooled with a cooling solvent (e.g., liquid helium) or in an ultra-low temperature freezer and used at an extremely low temperature, during which a current is supplied to the coil through a current lead made of a normal conductor. Therefore, evaporation of the cooling solvent and low operation efficiency of the freezer both due to a heat load on the normal conductor current lead pose problems. The heat load due to the current lead is roughly divided into an invasion heat from the outside that is transmitted through the current lead and Joule's heat generated in the current lead when a current is flown.
For these two kinds of heat loads to be reduced simultaneously, a superconducting oxide (O.sub.2 -treated conductor) fiber has been conventionally used as the material of a current lead (hereinafter this current lead is to be referred to as a superconducting oxide current lead). A superconducting oxide (O.sub.2 -treated conductor) shows heat conduction performance as that of ceramic, which is characteristically strikingly smaller than that of a metal. In terms of electric conduction, unlike a metal which conducts by the transmission of a free electron, since Cooper pair is involved, a flow of a current up to a certain level (critical current value) does not cause degradation of superconductivity in the absence of resistance. In other words, a flow of a current does not cause generation of Joule's heat. Therefore, a current lead composed of a superconducting oxide fiber can realize both superior electric conduction and reduction of invasion heat.
Bi superconducting oxide, which is easily produced, has been heretofore used as a material of a superconducting oxide current lead. However, since critical current density (maximum current flowable in a superconducting state) of a Bi superconducting oxide bulk is as small as approximately 1000 A/cm.sup.2, the resulting lead is required to have a large sectional area to accept a desired current. For example, when the current of 1000 A is to be flown, the lead should have a diameter of about 12 mm.
The present inventors have succeeded in obtaining a single crystal bulk of an RE123 superconductor (rare earth superconducting oxide) having a critical current density of 70,000 A/cm.sup.2 and produced by a unidirectional solidification method (Y. Imagawa et al., Physica C280 (1997) 255). Such a markedly high critical current density means capability of producing a superconducting oxide current lead that affords necessary current value with a small cross sectional area. For example, a current lead having a diameter of 1.3 mm and a smaller cross sectional area can accept a flow of the aforementioned 1000 A current.
The use of such an RE123 superconducting current lead enables reduction of a space. The amount of the heat that invades from outside through a current lead is in proportion to a cross sectional area and an inverse proportion to a length. Therefore, the use of an RE123 oxide superconducting current lead brings about further reduction of invasion of heat, thereby reducing the load on a freezer and further reducing the amount of evaporated cooling solvent.
Even when a superconducting oxide current lead is used, its terminal needs to be connected to a regular copper wire. However, for a high current to be flown, a terminal of a metal (normal conductor) should be attached.
In the case of a Bi superconducting oxide current lead, a terminal is formed by sputtering of a metal on a superconductor. In this sputtering method, the contact resistance at the interface between the superconducting oxide (O.sub.2 -treated conductor) and a terminal is as high as approximately 10.sup.-9 .OMEGA.m.sup.2 (J. W. Ekin et al. Appl. Phys. Lett 52 (1998) 331). A Bi superconducting oxide current lead does not suffer much from the contact resistance because of a large contact area between the terminal and the superconducting oxide (O.sub.2 -treated conductor), wherein less Joule's heat is generated due to the contact resistance at the interface.
When a terminal is formed in the same manner as in a Bi superconducting oxide current lead for an RE123 superconducting current lead, however, the contact area between the terminal and the superconducting oxide (O.sub.2 -treated conductor) is small due to less cross sectional area, which causes higher Joule's heat at the interface.
When the cross sectional area of a current lead is to be made smaller using a material having a higher critical current density, like an RE123 superconducting oxide current lead, the contact resistance at the interface between the terminal and the superconducting oxide (O.sub.2 -treated conductor) needs to be made smaller. For example, when an about 10 mm long metal terminal is to be attached to the terminal of a YBCO superconducting oxide current lead having a diameter of 1.3 mm, which is capable of flowing a 1000 A current, the contact resistance needs to be not more than 10.sup.-12.OMEGA.m.sup.2.
A superconducting oxide and a normal conductor (e.g., metal) have been conventionally joined by sputtering. In recent years, a molten method superior in the contact resistance at the interface has been proposed (Quarterly Journal of the Japan Welding Society, vol. 14, No. 1, pp.162-167 (1996)). The molten method comprises once melting a normal conductor and adhering the molten normal conductor to a superconducting oxide to give a terminal.
In the proposed molten method, however, the metal component (particularly Cu) of the superconducting oxide sometimes melts out into the molten normal conductor, thereby forming a reaction layer at the interface between the superconducting oxide and the normal conductor. Since this reaction layer is not a superconductor, the contact resistance between the superconducting oxide and the normal conductor becomes strikingly high.
It is therefore an object of the present invention to provide a method for manufacturing a joint, particularly a superconducting oxide current lead, having a less contact resistance at the interface between the metal terminal and the superconducting oxide (O.sub.2 -treated conductor). Particularly, to provide a method for manufacturing a joint of a superconducting oxide (O.sub.2 -treated conductor) and a normal conductor, which is capable of inhibiting formation of a reaction layer at the interface between the superconducting oxide and normal conductor.